The impact of transfusion-transmitted virus (TTV) infection on acute fulminant and non-fulminant hepatitis is unclear. In this study, serum samples from 164 patients with acute hepatitis of various aetiologies, from 34 asymptomatic hepatitis B virus carriers and from 202 healthy adults were tested for TTV DNA by the semiconserved nested polymerase chain reaction. TTV viraemia was prevalent in patients with acute hepatitis C, in patients with acute hepatitis D virus superinfection and in patients with non-A-E hepatitis (27-30%) but the incidence was not significantly different from that of healthy controls (31 of 202, 15.3%). There were no significant differences in gender, age, presence of hepatitis G virus, the occurrence of fulminant hepatitis, or in serum albumin, bilirubin or alanine aminotransferase levels (9/30 vs 35/134) between patients with or without TTV viraemia. Seven of the nine TTV-positive patients with fulminant hepatitis were co-infected with hepatitis C, D or E. TTV clones from 18 subjects, with or without fulminant hepatitis, were sequenced and analysed phylogenetically. Eleven (61. 1%) belonged to TTV group 1, six (33.3%) to TTV group 2 and one to TTV group 3. No particular strain of TTV was associated with fulminant hepatitis. In summary, in Taiwan, TTV is prevalent in the general population as well as in patients with liver diseases. TTV plays an insignificant role in acute fulminant and non-fulminant hepatitis. Its influence on outcome requires a longitudinal study.